1. Field of Invention
The present invention relates to an organic dye laser system employing a novel solid state periodic structure as a distributed-feedback host.
2. Background Information
Throughout this application, various publications and patents are referred to by an identifying citation. The disclosures of the publications and patents referenced in this application are hereby incorporated by reference into the present disclosure.
Since their discovery in 1966, organic dye lasers have been one of the most versatile coherent light sources. These lasers provide broad tunability over a spectral range that covers the ultraviolet (UV) to near-infrared (NIR) and their pumping methods are flexible. Dye lasers are highly efficient and they can be operated in either a continuous-wave (CW) mode with kilohertz linewidth or in pulsed mode with femtoseconds in duration. The cost of the organic dyes is negligibly small, compared to that of solid-state tunable lasers such as Ti:sapphire lasers. Dye lasers have been operated using solids, liquids or gases as the gain medium. Liquid dye lasers are especially popular because cooling and replenishing are achieved by a simple circulating system. A liquid gain medium is self-repairing, in contrast to a solid-state medium where damage is permanent. Despite these attractive attributes, liquid dye lasers remain problematic as they tend to be difficult to handle and many dyes and solvents raise health and environmental concerns.
Laser dye in a solid matrix has been actively developed as an alternative. Recent approaches to realize solid-state dye lasers include incorporating the laser dyes in polymers, such as polymethyl methacrylate (PMMA), sol-gel and organically modified silicates (ORMOSILs) as the host for the gain medium. Being in a solid-matrix, many of the problems associated with a liquid system are eliminated. However, solid-state dye lasers have been plagued by the photodegradation of laser dyes, as manifested in short operating lifetime. The photodegradation problem may be circumvented by a gain medium moving relative to the pump beam. Dispersive elements such as grating, prism or a combination of them within the optical cavity are typically used for wavelength tuning. While such intracavity configuration offers tuning flexibility, it generally requires a stable opto-mechanical alignment for optimal performance.
Mirrorless dye lasers with optical feedback distributed throughout the gain medium were first reported in 1971. (C. V. Shank, J. E. Bjorkholm and H. Kogelnik, `Tunable distributed-feedback dye laser,` Applied Physics Letters, 18, 152 (1971)). The distributed feedback (DFB) is obtained by a gain medium with a spatial modulation either in refractive index or gain in the direction of light propagation through the gain medium, i.e., in or parallel to the film plane. Periodic modulation in gain and index can be obtained, for example, by two interfering coherent pump laser beams on a dye-doped film. The laser emission is in, or parallel to, the film plane, normal to the periodic structure and the output wavelength is set by the periodicity. By either varying the angle between interfering pump beams or the refractive index of the dye solvent, the dye laser can be tuned. DFB dye lasers, in which the gain medium, laser cavity and wavelength tuning elements are combined into a thin film, offer potential advantages. However, photodegradation remains problematic as such DFB dye lasers tend to be difficult to implement with a moving gain medium, thereby making them impractical.
Goldberg et al, in U.S. Pat. No. 3,771,065, entitled `Tunable internal-feedback liquid crystal-dye laser,`filed Nov. 6, 1973) and later Il'chishin et al (I. P. Il'chishin et al, `Detecting of the structure distortion of cholesteric liquid crystal using the generation characteristics of the distributed feedback laser based on it,` Molecular Crystals and Liquid Crystals, 265, 687 (1995), and I. P. Il'chishin et al, `Generation of a tunable radiation by impurity cholesteric liquid crystals,` JETP Letters, 32, 24 (1980)), have disclosed dye lasers with cholesteric liquid crystal (CLC) as a host that provides distributed feedback. Typically, the dye-doped CLC is confined between two flat glass substrates. The CLC is aligned in the so-called planar texture, resulting in a phase grating through the CLC layer. The laser emission is normal to the film plane and the output wavelength is set by the helical periodicity. By varying the temperature of the CLC host, the helical pitch of the CLC host can be changed, thereby the output wavelength of the dye laser can be tuned. However, the operation of these dye lasers which utilize CLC hosts in fluid form is subject to environmental perturbation, such as temperature. In addition, the photodegradation is still problematic as these DFB dye lasers are difficult to implement with a moving gain medium, thereby making them impractical for many applications.